1. Field of the Invention
The invention relates to a steering system.
2. Description of Related Art
There is a conventional electric power steering system (hereinafter, referred to as “EPS”) that assists a driver's steering operation by applying torque generated by a motor to a steering mechanism of a vehicle. For example, an EPS described in Japanese Patent Application Publication No. 2011-88462 (JP 2011-88462 A) includes a rack-and-pinion mechanism that serves as a steering mechanism. The rack-and-pinion mechanism changes the orientation of wheels by converting the rotation of a pinion caused by a steering operation into a liner motion of a rack shaft (steered shaft) meshed with the pinion. The rack shaft is provided with a ball screw mechanism that converts the rotation output from a motor into a linear motion of the rack shaft. That is, the torque generated by the motor is converted into an axial force of the rack shaft in its axial direction, so that the steering operation is assisted.
Specifically, as illustrated in FIG. 9, in an EPS 100, the torque generated by a motor 101 is transmitted to a rack shaft 106 via a drive pulley 102, a timing belt 103, a driven pulley 104 and a ball screw mechanism 105. The opposite ends of the rack shaft 106 are connected to wheels via tie rods (not illustrated).
The ball screw mechanism 105 includes a ball nut 107 screwed into a spiral groove 106a in the rack shaft 106 via multiple balls. A first end portion of a cylindrical nut holder 108 that covers the outer periphery of the rack shaft 106 is fitted to the outer peripheral face of the ball nut 107, and the driven pulley 104 is fitted to the outer peripheral face of a second end portion of the nut holder 108.
The ball nut 107 is held between a step portion 108a formed in the inner peripheral face of the nut holder 108 and a cylindrical external thread 109 screwed to an opening portion of the nut holder 108, which is located on the opposite side of the nut holder 108 from the driven pulley 104. Thus, an axial displacement of the ball nut 107 and rotation of the ball nut 107 relative to the nut holder 108 are restricted. A bolt 110 is screwed into the nut holder 108. The distal end of the bolt 110 is passed though the nut holder 108 in the radial direction of the nut holder 108, and is fastened to the outer periphery of the ball nut 107. Thus, even if the external thread 109 is loosened, the rotation of the ball nut 107 relative to the nut holder 108 is restricted.
Three bearings 111, 112, 113 are interposed between the outer peripheral face of the nut holder 108 and the inner peripheral face of a casing 115. The nut holder 108, the ball nut 107 and the external thread 109 rotate together with each other in accordance with the rotation of the driven pulley 104.
In recent years, energy saving effect of EPSs including the EPS described in JP 2011-88402 draw widespread attention, and therefore the number of types of vehicles in which EPSs are mounted has been increasing. On the other hand, there has been a growing demand for more compact EPSs from the viewpoint of enhancing the ease of mounting the EPSs in vehicles. However, in the EPS 100 described in JP 2011-88462 A, the driven pulley 104, the two bearings 111, 112, the bolt 110, the ball nut 107 and the external thread 109 are arranged on the nut holder 108 along the axial direction of the rack shaft 106. Thus, there are limitations to reduction of the length of the EPS 10 in the axial direction of the rack shaft 106.